1. Field of the Invention
The present invention relates to a method and an apparatus for evaluating immunity characteristics of an electronic circuit device or an electronic circuit component, such as an LSI or a memory, to electromagnetic noises such as conductive noise and radiation noise and to an LSI tester having those evaluation functions.
2. Description of the Prior Art
Electromagnetic noises received by electronic equipment or electronic circuit devices are classified into radioactive noises caused by a radiation field in the air emitted from radiobroadcasting equipment, televisions, and mobile telephones and conductive noises conducting through power lines, signal lines or ground lines to be connected to electronic equipment or circuit devices. Recently, however, there has been occurred a problem that these electromagnetic noises may cause malfunction of digital equipment having LSIs or memories.
Therefore, conventionally there has been known an immunity evaluation device for evaluating resistance to noises of electronic equipment, or printed circuit board mainly evaluating behaviors of malfunction of the digital equipment by giving an electromagnetic noise to the equipment from its outside. While this method is suitable for checking a degree of intensity of the noise which causes the malfunction of the digital equipment, it is difficult to specify which circuit device or printed circuit board has malfunctioned in the equipment. To improve noise immunity which is a performance of resistance to noises of digital equipment, however, it is important to evaluate noise immunity of a single circuit device such as an LSI and to specify a cause of malfunction of the digital equipment.
On the other hand, as a device for use in evaluating an LSI function, conventionally an LSI tester has been used in general. The LSI tester is used for checking an operation at manufacturing an LSI by previously obtaining input-output characteristics assumed based on design uses in order to evaluate whether or not the manufactured LSI satisfies the characteristics. A principle of the conventional LSI tester is shown in FIG. 1.
In this LSI tester, as shown in FIG. 1, an input pattern 1 which is an input signal group required for a normal operation is applied to each input terminal of an LSI 4 which is to be evaluated and then an output pattern 2 which is an output signal group obtained by an output terminal as a result is compared with an expected pattern 3 which is an output group which is previously expected at a normal operation, by which it has been judged that the LSI is normal if all the patterns match and that it is abnormal if any one of the parts of the pattern does not match the other one. The abnormal state shows that the target LSI is in error because of any fault of LSI caused in its manufacturing process. This LSI tester is characterized in that it can evaluate an LSI singly. The LSI tester, however, is intended for judgment of an error due to any fault of LSI and not for evaluating noise immunity, which shows a maximum noise level that causes malfunction in an LSI.
Generally speaking, there are two types of malfunction; one is caused by an application of conductive noises which had been entered through wire patterns to the LSI and the other is caused by radiation noises in the air which the LSI has directly received. When the LSI immunity is evaluated, these two types of malfunction must be evaluated; the former is an immunity evaluation to the conductive noises and the latter is an immunity evaluation to the radiation noises.
As a test equipment which can be used for the immunity evaluation of electronic circuits to the conductive noises, there is known a test equipment disclosed in Japanese Non-examined Patent Publication No. 4-95786, for example. It is an immunity evaluation equipment for an electronic circuit comprising a plurality of components mounted on a printed circuit board. This test equipment is characterized in that an LSI can be evaluated while is mounted on the board together with other components.
As shown in FIG. 2, the conventional test equipment comprises an electromagnetic radiator 16, a rod antenna 17, and an electromagnetic shielding box 23. A circuit component 25 to be evaluated is mounted on a circuit board 18. When electromagnetic waves are radiated from the electromagnetic radiator 16 with the rod antenna 17 in contact with a terminal 20 of the circuit component 25 to be evaluated, the magnetic waves are received by the rod antenna 17, and then noise current induced by the rod antenna 17 is applied to the circuit component 25 on the circuit board 18 via the terminal 20. By confirming that the circuit does not malfunction at this point, the noise immunity of the circuit component 25 can be evaluated. By using this equipment, it is possible to apply an electromagnetic noise selectively only to the terminal 20 of the circuit component 25 to be evaluated, and to evaluate selectively and accurately a circuit which is not resistant to radioation noises under actual use conditions since malfunction easily occurs in circuits having high input impedance.
The conventional immunity evaluation method in the above, however, is intended for evaluating noise immunity of an electronic circuit board on which various LSIs and circuit components are mounted, and not necessarily for evaluating noise immunity of a single electronic circuit component to which a noise is applied. Particularly in a digital circuit, an operation is normally progressed by an applied program, and in some cases the operation is put in an off state on the program even if the circuit is physically connected depending on an execution part of the program. In other words, even if it is connected in hardware, it may be disconnected in software. If an electromagnetic noise is applied in this state, the entire circuit does not malfunction. In other words, immunity significantly depends upon software operation modes, on whether or not an applied electromagnetic noise causes malfunction of the entire circuit.
In addition, an electric field in the electromagnetic shielding box is not always uniform and therefore the result of the evaluation may be significantly different from that of an immunity evaluation method with an antenna which is applied in a radio shielding room in order to evaluate noise immunity by irradiation with plane waves.
Furthermore, generally it is not easy to identify the operation mode while an actual electronic circuit board is in operation.
In the conventional immunity evaluation method in which an electronic circuit board is used, a single electronic circuit component such as an LSI cannot be sufficiently evaluated on its immunity.
On the other hand, as to a general method of evaluating characteristics of noise immunity to radioactive noises, as shown in FIG. 3, an electronic circuit component or an electronic circuit board 21 to be evaluated is placed in an operating state in an electromagnetic echoic chamber 19, and then the electronic circuit component or the electronic circuit board 21 to be evaluated is exposed to electromagnetic waves W from an antenna 23 put in a position spaced a given distance, for example, 10 m away from the evaluation target in order to check the presence or absence of malfunction of the evaluation target. According to this method, each component can be exposed to electromagnetic waves in a plane wave mode, that is, a TEM mode so as to be evaluated under uniform conditions, and therefore it is a suitable method for an evaluation of a technical standard, by which the method is the most popular for an immunity test of a product. In an evaluation with this method, however, it is not easy to specify which circuit component has malfunctioned at an occurrence of an erroneous operation, and therefore it is not suitable for an immunity evaluation of a single circuit component.
Supposing that it is attempted that an electromagnetic noise is applied in an antenna method with an LSI tester which allows a single LSI to be evaluated as a circuit component, it is necessary to place the LSI tester itself in a radio shielding room. The LSI tester, however, which is generally large-sized, cannot be easily placed in the electromagnetic echoic chamber. Even if it can be placed in the electromagnetic echoic chamber, the LSI tester itself is caused to receive electromagnetic noises, by which the noise immunity of the target LSI may not be evaluated correctly.